Rust forms on steel structures such as bridge, plant, ship, and building with lapse of time. Accordingly, corrosion resistant alloy steels such as weathering steel with retarded corrosion speed are recently used for the members of steel structures such as bridge. However, thick, high density, and adhesive layered rust and imbricate rust are formed under some environmental conditions to which they are exposed. The rust invites deterioration of the steel structure, and therefore, the life of the steel structure should be elongated by coating the steel structure after removing the rust. Early removal of the rust and subsequent coating of the steel matrix are required especially when the rust is thick since the steel structure has the risk of suffering from safety problems during its use.
In the steel making, a large amount of steel casting is sometimes stored in the exterior for prolonged time according to the production plan. Since steel making are normally located in the seaside area, thick rust is likely to be formed on the steel casting by the airborne salt grains wafting from the ocean. In such a case, the thick rust should be removed before subjecting the steel to the hot rolling step since the rust results in the surface defects and scabs which result in the loss of commercial value.
However, complete removal of the rust formed on the steel structure or the steel casting is technically an extremely difficult task, and a large noise is usually generated in the removal of such thick rust and heavy burden is placed on the operator.
For example, alumina- or silicone carbide-based grinders and paper grinders have been used for the removal of the rust formed on the steel material. However, when the rust is thick, high density, and adhesive, grinding of such rust having a hardness higher than the alumina or the silicon carbide is difficult by using such material for the grinding.
The thick and firm rust may also be removed by a power tool such as jet chisel. However, this method which is capable of conducting rough grinding is incapable of conducting the precise grinding. More specifically, removal of the rust and exposure of the steel matrix to a degree sufficient for the subsequent coating is difficult when thin rust has firmly deposited on the steel surface. Also, the loud noise in this process is a great burden for the operator.
Also, the rust may be removed by blasting. Blasting, however, has the problem of terrible noise and it also requires large scale apparatus and high cost.
In view of such situation, the inventors of the present invention proposed, in Patent Document 1, a rotary grinding tool capable of removing thick and firmly bonded rust on a steel structure with a large surface area such as bridge in an effective, efficient, and convenient manner at high speed and low cost, with high safety and workability. This rotary grinding tool has high rust removing and surface exposing ability and this tool can conduct the rust removal and the steel surface exposure at once. In this rotary grinding tool, hard grains having a particular hardness are provided on the grinding surface of the metal rotary disk at a particular surface density so that the grains are exposed to a certain extent. Patent Document 1 discloses a curved rotary grinding tool having a grinder disk surface including the part where the angle between the normal line of the grinder disk surface and the rotary axis is in the range of at least 1° to up to 45°, and the grinder peripheral surface includes the part where the cross-section parallel to the rotational center has a radius of curvature R of at least 1 mm and up to 10 mm. The only embodiments disclosed in Patent Document 1 are those having such curved grinding surface.
Similar rotary grinding tool is disclosed in Patent Document 2. The Patent Document 2 proposes a diamond grinder disk having a plurality of diamond grain pieces secured to the surface of the disk having the grinding function. In this diamond grinder disk, the distance between two adjacent diamond grain pieces on a particular rotation track is larger than the distance between the diamond grain piece on the particular rotation track and the nearest diamond grain piece on the rotation track radially adjacent to the particular track. Patent Document 2 describes that such diamond grinder disk can be used with no substantial difference from conventional commercial products; all diamond grain pieces contributes efficiently and equally to the grinding process; the diamond grain pieces are unlikely to experience uneven abrasion even after prolonged use; and grinded rust is smoothly discharged from the center to the periphery of the disk surface.